The present invention relates to doppler tracking systems and more particularly, to doppler tracking systems where interference signals are suppressed and a signal for tracking is generated.
Heretofore, doppler tracking systems have been highly susceptible to velocity deception jamming. Velocity deception jammers (sometimes called velocity-gate stealers) intercept the transmitted radar signal, introduce a frequency shift, and retransmit it at a very high power level. Initially the frequency shift is small so that the jamming signal enters the passband of the doppler filter of the victim radar and captures its doppler-tracking AFC loop. Then the frequency shift is increased and, since the AFC loop is now tracking the stronger jamming signal, it is pulled away from the target doppler signal. After sufficient separation is achieved, the jamming signal is turned off or its program is recycled, so that the AFC loop no longer has a signal to track. It must then attempt to reacquire the target doppler signal. It may fail to do so, or it may lock onto another deception signal transmitted by the jammer.
Various attempts have been made to counter velocity jamming. For example, sensing the presence of the jamming signals and opening the AFC loop for as long as the jamming signal is in the doppler filter passband; limiting the tracking capability of the AFC loop so that it will not follow jamming signals that are rapidly changing frequency; and providing information to the AFC loop from an external source to assist in maintaining lock on the target doppler signal. The first example has the disadvantage that the doppler signal from a maneuvering target can leave the doppler filter passband while the AFC loop is open. The second example essentially defeats the purpose of the AFC loop in that it will not be able to track highly maneuverable targets and could still track jamming signals that are of slowly changing frequency. The third example has the serious disadvantage of requiring a second radar.